What Is a Yeast Infection?

Most women — and some men — will have a yeast infection at some point in their lives.

A vaginal yeast infection, also called candida vaginitis or vulvovaginal candidiasis, is a vaginal infection resulting from the overgrowth of yeast (a type of fungus).

Yeast infections are most commonly caused by the fungus Candida albicans, but are also infrequently caused by other Candida species, including C. glabrata, C. parapsilosis, C. tropicalis, and C. krusei.

Up to 75 percent of women will get a vaginal yeast infection at least once in their lives, and 40 to 50 percent of women will experience more than one infection, according to a 2007 article in the medical journal The Lancet. What's more, 5 to 8 percent of women suffer from recurrent or chronic vulvovaginal candidiasis, wherein they come down with four or more yeast infections in a single year.

Though the term "yeast infection" is most often used to refer to a vaginal infection, it also applies to other types of candidiasis. A yeast infection of the mouth is called thrush, or oropharyngeal candidiasis. A yeast infection of the skin — which typically occurs in warm, moist areas, such as the armpits and groin — is called cutaneous candidiasis.

And if Candida gets into the bloodstream, such as from using a contaminated intravenous catheter, the yeast can cause a deadly infection called invasive candidiasis.

Symptoms of Yeast Infections

White or grey vaginal discharge that may be slightly watery, or thick and chunky like cottage cheese

Pain during vaginal sex

Redness and swelling of the vulva

Men, particularly those who are uncircumcised, can get a form of penile yeast infection called candidal balanitis. These infections usually spring up in men who have unprotected sex with a woman who has a yeast infection, though it's not considered a sexually transmitted disease because it can also develop in the absence of sexual activity.

Symptoms of a yeast infection in men most often include a red rash on the penis and an itching or burning sensation on the tip of the penis.

Causes of Yeast Infections

Candida albicans and other Candida species are a normal part of the microbiome — the community of microorganisms that live inside our bodies and on our skin.Candida fungi only cause an infection when something throws off the balance between the various members of your microbiome, causing Candida numbers to increase, such as:

Pregnancy

Certain medications, including birth control pills, antibiotics, and steroids

Wearing clothing that keeps the vaginal area warm and moist, such as synthetic underwear, pajama bottoms, and tight jeans or spandex

Yeast Infections During Pregnancy

Yeast infections are a common occurrence during pregnancy, when "the micro environment of the vagina changes to some extent in response to the estrogen levels," explains Gregory R. Moore, MD, MPH, an obstetrician-gynecologist and director of the University Health Service at the University of Kentucky in Lexington.

“Yeast likes warm, moist, airless environments. Being pregnant and wearing support hose or constrictive clothing can make the vaginal environment all the more warm, moist, and dark, encouraging yeast to grow,” Dr. Moore says.

If you experience unusual vaginal discharge during pregnancy, you should see your doctor to exclude bacterial vaginosis or sexually transmitted infections rather than assume it is a yeast infection. Chlamydia, gonorrhea, and trichomoniasis will not respond to treatments for vaginitis and can cause complications during the pregnancy.

"There has not been adequate human-pregnancy data to make a decision one way or another on the effect of oral anti-fungal medications on the fetus," Dr. Moore says.

While yeast infections generally don't put a pregnancy at risk, they can be very uncomfortable. If you think you have a vaginal yeast infection during pregnancy, it should be diagnosed and treated by a doctor.

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